This invention relates to facsimile equipment and, more particularly, to the generation of frequency modulated facsimile signals representing dark/light variations (or ligh/dark variations) on a document. Copending application Ser. No. 844,298 filed 10/21/77 discloses a facsimile transmitter which synthesizes a frequency modulated sinewave representing dark/light variations on a document from a high frequency pulse train which is frequency modulated to represent the same dark/light variation. This type of transmitter is particularly advantageous since it allows the frequency of the sinewave to change rapidly without delays and phase distortion substantially instantaneously with a change in the frequency of the pulse train. More particularly, the sinewave is capable of changing frequency in a mere fraction of its cycle in response to a change in pulse frequency. This may be achieved with minimum filtering since high frequency harmonics which must be attenuated are substantially higher in frequency than the output signal, thus the phase delays and distortions which would otherwise be introduced with filtering are avoided.
The synthesizing of a sinewave for use in connection with facsimile equipment is particularly desirable at this time in view of the liberalization of the rules by the FCC which permits direct connection of customer-provided equipment including facsimile transceivers to the telephone network without benefit of acoustic coupling which can itself introduce distortion. However, the liberalized rules for direct connection to the telephone network still require certain limitations in the frequencies which may be introduced into the telephone network. Accordingly, it is generally desired to maintain the frequency of the transmission signal below a maximum upper frequency, e.g., 2525 Hz. for standard voice grade telephone network, since there is no low pass filter associated with the transmitted signal. However, there are no means provided in such a digital-to-analog synthesizing network to assure that the frequency will not rise above 2525 Hz.